In conventional telephone systems of the common control type, for providing interconnections between the common control PABX and a central office, a large number of trunk circuits for the PABX are employed and the individual trunk circuit cards usually include both latching circuitry and all the control logic necessary for controlling the operation of the trunk. For controlling a trunk connection, the system monitors and operates on the sleeve lead and connections between a central office and the PABX are carried out on a per trunk basis.
In our co-pending application entitled "MATRIX-PBX" Ser. No. 431,928, filed Jan. 9,1974, and assigned to the assignee of the present application, now U.S. Pat. No. 3,943,297, there is disclosed a new and improved type of common control PABX, in which the switching matrix is completely independent of signalling between the line circuits and the common control with regard to line conditions and maintainence of the communication condition. Instead, the switching matrix is employed only for talking purposes and the common control is continuously associated with all circuits, so as to monitor the condition of each calling and called circuit. This operation is considerably different from conventional control functions provided in a common control PABX, wherein the common control is used to set up the call through the matrix and then drops out after the connection has been established. As a result, the trunk circuits for conventional systems are quite complex and, because the systems operate on a per trunk basis, a considerable number of trunk circuits are employed for the various types of system connection capabilities required.